In order to show some image on a cathode ray tube (CRT) screen whether it be text or graphics, the CRT beam must be turned on (brightened in the case of the image being bright and the background dull) at the correct time to produce a dot, or a series of dots to make a bar, and the like. The foregoing necessitates that the display system have a data storage means which transmits data signals, representing the image to be displayed, in synchronism with clock signals from a clock signal generator. In the prior art, the clock generator is usually located in the CRT circuitry. In synchronism with the clock signals, the CRT beam is turned on (or not turned on) as the beam gets to the correct data location. It should be understood that in the case of the intelligence being in a dark mode, against a bright background, then the beam would be turned off. All of the foregoing is well understood in the prior art.
Further in the major part of the prior art, if a system is going to display both graphics and text, two different memory and control systems have been employed. It has been only when the information is finally transmitted to the CRT device that the graphics and text signals have been meshed, or multiplexed.
The introduction of the bit map memory, to the display art, has made the task somewhat easier because in a bit map memory, there is a memory for each pixel location on the CRT screen. Hence, whatever image is to be seen (either text, or graphics, or a combination of both) can be written into the bit map memory and from that one memory (as it is scanned) both graphics and text data signals can be transmitted to the CRT screen to be shown.
The major problem that occurs in the prior art is that the bit map memory was not used for text but the text was encoded from a character generator and continually transmitted to the CRT. While the prior art system was fast, it required two separate memory systems. The present system permits both text and graphics to use the same memory while operating with the speed of the separate memories of the prior art. The present invention employs a five-fold improvement in transmitting data signals to the data display device as will explained below.